The invention relates to state estimation of vehicles such as high-altitude aircraft, spacecraft and satellites. More particularly, the invention relates to an attitude determination and alignment system for vehicles such as low-earth orbiting spacecraft or high-flying aircraft using electro-optical sensor devices and navigation satellites in a global positioning satellite system.
Spacecraft, aircraft, and satellites must accurately determine absolute orientation (i.e., pitch, yaw, and roll) to realign their onboard Attitude Control System (xe2x80x9cACSxe2x80x9d) and correct for instrument drift and buildup of errors affecting accurate and precise attitude determination. Attitude control is of particular importance in aircraft to maintain a stable operating environment, in surveillance satellites to track another object in space, and in remote imaging satellites to provide precise earth imaging and reconnaissance.
Prior methods have been developed to perform ACS functions. For example, one such method measures angular positions of stars and compares those measurements to known quantities. Kainel, U.S. Pat. No. 5,963,166, provides a spacecraft camera image navigation and registration system to point a satellite and camera. An onboard computer performs calculations based upon information from a star tracker, gyro, and earth-based sensor data to determine the attitude of the satellite. However, Kamel requires the use of star tracker equipment to detect stars in the ever-changing area above the satellite. The star tracker compares detected star positions with a star table and then determines vehicle attitude from the detected star positions. While a star tracker provides attitude accuracy on the order of 5 to 20 xcexc radians, the star tracker equipment required to detect the faint star light and maintain the star tables is costly.
Ring, U.S. Pat. No. 5,959,576 provides a satellite attitude determination system using a global positioning system (xe2x80x9cGPSxe2x80x9d) and line of sight communication instead of star tracker equipment. Ring provides a dual-axis pointing laser receiver on one satellite and laser transmitters on other satellites to determine relative azimuth and elevation. The relative orientation is combined with Global Navigation Satellite System (xe2x80x9cGNSSxe2x80x9d) position data to determine the attitude of the satellite. Ring provides attitude determination for a communication satellite without the use of star tracker equipment. However, Ring""s design concept provides coarse attitude accuracy using radar signals, not suitable for applications requiring greater accuracy, such as remote imaging or surveillance satellites. Although it can provide accurate attitude determination using laser communication links, it is potentially very complex and costly.
Van Dusseldorp, U.S. Pat. No. 5,943,008, exemplifies an attitude determining system utilizing a GPS. According to Van Dusseldorp, at least three sets of signals are respectively received from three antennas onboard a vehicle. Each signal is received in a separate time domain slot, with each signal respectively receiving information from a respective satellite on a separate dedicated channel.
However, multiple antenna GPS systems provide relatively coarse attitude accuracy, on the order of 1 milli-radian at best, and therefore may not be used in applications requiring greater accuracy, such as in remote imaging satellites or surveillance satellites.
Techniques such as those described above and existing interferometric GPS attitude determination methods using multiple antennas are complex and expensive, or provide only coarse information with milli-radian accuracy.
Recently, the U.S. Air Force has completed deployment of the Global Positioning System (xe2x80x9cGPSxe2x80x9d) Block II constellation of 24 satellites. Russia has similarly deployed a global navigation satellite system GLONASS in a similar orbit constellation. Currently 10 of 18 satellites are operational with a full constellation being 30 satellites. European Space Agency (xe2x80x9cESAxe2x80x9d) is scheduled to develop yet another GPS/GNSS, based of a constellation of 30 Galileo satellites. The position and motion of these satellites, by the very nature of their mission, are known to high precision and the satellites are uniformly distributed about the celestial sphere relative to the center of the earth. If the relative angular position of a navigational satellite can be measured relative to spacecraft axes then these satellite constellations can be used as surrogate calibration stars for the purpose of the attitude determination and alignment updates.
The design approach introduced herein extends the capability of the GPS/GNSS to perform satellite attitude calibration using an on-board electro-optical sensor to determine the three-axis alignment errors of the spacecraft ACS. This design replaces a traditional star tracker, eliminating the need for maintaining data memory for a calibrated star catalog. As illustrated and summarized in FIG. 1, the present invention using a Navstar GPS inertial navigation system (xe2x80x9cINSxe2x80x9d) works as follows:
A spacecraft vehicle equipped with a GPS receiver unit has a worldwide navigational capability to compute an absolute position and velocity with an instantaneous accuracy of less than 16 meters (SEP) and 0.1 meter/second (1"sgr"), without the aid of additional instruments or external reference sources. Also, by extracting the ephemerides of the GPS satellites contained in the GPS broadcast signal, the location of each GPS satellite is available with an accuracy of 8 meters (SEP). The accuracy of the data can be improved to one meter or less by implementing an integrated Kalman filter solution over a time period. Having very precise and accurate knowledge of both the vehicle and all GPS satellites in view, the pointing vector to each GPS satellite can be computed versus time to sub-microradian accuracy.
Nominally, two GPS satellites, with a sufficient geometric separation, are selected for pointing an on-board visible or equivalent electro-optical sensor. The GPS satellites can be simultaneously or sequentially viewed and tracked by the sensor over a nominal time period.
The sensor is pointed by reorienting the sensor line-of-sight with respect to on-board attitude data derived by an inertial measurement unit (xe2x80x9cIMUxe2x80x9d) or similar measurement device, such as a 3-axis magnetometer. This may be accomplished in several ways. FIG. 8 illustrates a sensor on a gimbaled mount. Reorienting the vehicle is required when using a strap-down sensor. Yet other approaches are also applicable, such as mounting multiple strap-down sensors is particularly illustrated in FIG. 1. For operation in the strap-down sensor configuration, a predication must be made of when satellite viewing will occur. For optical or ultraviolet viewing of each GPS satellite, the satellite is allowed to streak across a camera in the form of a sensor focal plane array.
The present invention provides attitude determination and alignment of a moving vehicle using electro-optical sensors and global navigation satellites without use of star tracker equipment. An attitude determination and alignment system for low earth orbit (xe2x80x9cLEOxe2x80x9d) satellites and high-flying aircraft determines vehicle orientation within micro-radians and at a significantly lower cost than comparable star tracker equipment. The present invention extends capability in existing global navigation satellite systems, such as Navstar GPS, that perform inertial navigation and timing by implementing a unique attitude determination and update technique. The present invention includes one or more electro-optical sensors to view and measure navigation satellites as surrogate calibration stellar sources. The use of a star catalog, required by star tracker systems, is replaced by computing real-time pointing vectors to the navigational satellites. The pointing vectors are computed by projecting accurate inertial positions of the subject vehicle, equipped with a GPS/GNSS receiver and an inertial navigation system (xe2x80x9cINSxe2x80x9d), and navigational satellites. System performance of the present invention is related to measurement precision of sensor instruments aboard the vehicle. Hence, a pointing accuracy on the order of a few micro-radians can be achieved even without the use of star tracker equipment.